Method for Producing a Sandwich Structure, Sandwich Structure Produce Thereby and Use Thereof

ABSTRACT

The invention relates to a method for producing a sandwich structure on the basis of at least one layer on the basis of metallic material and on the basis of at least one layer of organic polymer, wherein for coating of at least one metallic surface with at least one metallic layer to be combined with the layer on the basis of organic polymer, an aqueous conversion composition on the basis of zinc, additional cations, poly(acrylic acid), and optionally silane, is brought into contact, wherein the liquid film thereby produced is dried on and wherein the metallic layer coated in such manner is brought into contact with at least one layer on the basis of organic polymer and is combined into a sandwich structure by means of compaction under pressure and/or temperature. The invention also relates to such sandwich structures.

The invention relates to a method for producing a sandwich structure on the basis of at least one layer on the basis of plastics with at least one layer on the basis of metallic material, for example sheet steel, wherein a special adhesion-promoting and conversion-protecting coating is undertaken on at least one surface, the sandwich structure produced in this manner and the use of the sandwich structure produced in this manner.

In automobile production and aircraft construction, but also in many other construction types, metallic components are nowadays used, in particular, as supporting parts. However, the weight of such components is large and is to be further reduced in a number of uses.

One route to weight reduction is the use of sandwich structures in which a part of the usually heavy but typically mechanically very stable metallic material is replaced with organic material and with which a well-adhering composite in a stable construction can be used.

Particularly preferred sandwich structures are, for example, those on the basis of metal-plastics, metal-plastics-metal or metal-plastics-metal-plastics-metal.

The coating of metallic surfaces for reasons of corrosion protection and/or paint adhesion is basically known from many publications. For this purpose, above all, conversion coatings are used. Herein, it is above all decided whether passivations (=treatments) which are intended to protect primarily a paint layer against corrosion in the long term or whether pre-treatments should be used before a further coating, for example, a paint or adhesive.

EP 1 651 432 B1 teaches a metal laminate which comprises between two outer metal sheets an adhesive polymer layer comprising polyamide, a copolymer of ethylene and an unsaturated carboxylic acid and/or a derivative thereof and a reactive copolymer which comprises a styrene maleic anhydride copolymer, which has a molecular weight from 1400 to 10000.

DE 102007046226 A1 relates to a composite component consisting of a first and second metal sheet with at least one polymer foam layer arranged between them.

DE 2627681 A1 teaches phosphating methods on the basis of acidic metal phosphate solution in the presence of a water-soluble polymer with monomer units of (methyl)acrylic acid and/or (meth)acrylamide.

U.S. Pat. No. 3,721,597 relates to production methods of laminates on the basis of aluminum panels, ethylene-acrylic acid copolymer and HD-PE as the inner layer.

As compared with single or multi-layered paint coatings which often have a thickness of the individual paint layer on metal substrates in the region of 1 to 50 μm, one or more layers of organic polymer (=polymer layer) which are used for a sandwich structure and are glued on as a layer and/or are pressed onto one another, require a significantly greater thickness than a paint layer, for example, a thickness in the region of 0.1 to 5 mm.

A high degree of adhesion between the individual layers of a sandwich structure is necessarily required for the production, in particular, of a stiffness-optimized sandwich structure, for example, a metal-plastics-metal sandwich structure. Previously, it has often been realized by means of a wet chemical application of a thin surface-activating and/or adhesion-promoting intermediate layer on the metallic surface of a metallic layer and the melting on, in particular, of thermoplastic components of a polymer layer. In order to apply the surface-activating and/or adhesion-promoting intermediate layer onto the metallic surface of a metallic layer, separate plant passes for pre-treatment, for example, of a metal sheet in a coil coating plant are required. For example, galvanized steel sheets, depending on their production method, for example, hot-dip galvanized steel sheet Z or electrolytically galvanized steel sheet ZE, require different process control, which entails additional effort. Other metal substrates can be created with the conventional method sometimes only with insufficient adhesion of the layers of the sandwich structure. Whereas on similar metallic surfaces as on ZE, a certain degree of adhesion has been achieved by means of an acidic conversion composition to form a conversion layer with approximately 30 mg Ti per m², on Z, an insufficient adhesion has been achieved. A sufficiently high level of adhesion could only be achieved on Z when by additional process steps with a first alkaline conversion composition, furthermore with additional rinsing with water and then with a second acidic conversion composition, a second conversion layer having approximately 30 mg Ti per m² was formed. When the manner of zinc coating was changed from time to time, this was an unacceptable disadvantage.

Furthermore, the conventionally used technique had the disadvantage, in particular, for hot-dip galvanized steel sheets, that the metal-plastics composite had only a limited adhesion in a peel strength test.

For this reason, further wet-chemical conversion compositions should be investigated. It would be advantageous if the conventional wet-chemical coating method for surface activation and/or adhesion-promotion could be more effectively adapted to the plastics used. It was an aim to ascertain alternative wet-chemical conversion compositions for all common metal substrates and, in particular, for metallic-coated steel substrates. It was also an aim to provide metallic layers for a sandwich structure with the aid of a thin adhesion-promoting conversion coating which serves as an intermediate layer of a sandwich structure and hereby to achieve a sandwich composite adhesion of at least 300N/4 cm at 4 cm sample width, which results in 75N/cm in the peel strength test according to ISO 11339, 2010.

It was an object to improve the adhesion between the surface of a layer on the basis of organic polymers, for example, a KTL temperature-stable polymer layer and the surface of a metallic layer, for example, as a cover panel, in order to produce a sandwich structure, in particular on the basis of metal-plastics, metal-plastics-metal, plastics-metal-plastics, metal-plastics-metal-plastics or metal-plastics-metal-plastics-metal. Sandwich structures, in particular, stiffness-optimized metal-plastics-metal sandwich structures, as planned under the trademark Litecor®, are particularly preferred.

Furthermore, it would be advantageous if the process complexity of the existing method for manufacturing sandwich structures could be improved by reducing the number of plant passes and if the bath stability for preparing the metallic surface and/or for adhesion-promotion could be increased and if the number of treatment steps necessary for particular metallic surfaces could also be reduced.

It has surprisingly been determined that with a special conversion composition, a single-step and, in particular, economical conversion coating can be carried out for all the technically relevant metallic surfaces, which leads to a sufficient, or even to an unexpectedly high level of adhesion of the metal-plastics composite.

The object is achieved with a method for producing a sandwich structure from at least one layer of metallic material and at least one layer of organic polymer, characterized in that at least one surface on at least one metallic layer which is to be combined with at least one layer of organic polymer is brought into contact with an aqueous conversion composition which contains:

-   -   0.5 to 20 g/l zinc,     -   0.01 to 10 g/l manganese, 0.01 to 10 g/l aluminum, 0.01 to 1 g/l         chromium(III), 0.01 to 5 g/l iron(II), 0.01 to 5 g/l iron(III)         and/or 0.01 to 5 g/l magnesium,     -   0 or 0.01 to 5 g/l of the total as nickel and/or cobalt,     -   0 or 0.01 to 5 g/l of the total as molybdenum, tantalum,         vanadium and/or tungsten,     -   2 to 100 g/l P₂O₅, which corresponds to 2.68 to 133.8 g/l PO₄,     -   0.1 to 10 g/l polyacrylic acid, but not more than 25% of the         content of the composition of P₂O₅ in g/l, and     -   0 or 0.01 to 3 g/l silane, but not more than 25% of the content         of the composition of P₂O₅ in g/l,         in that the liquid film produced therewith is dried on, in that         the metallic layer coated in this manner is cut, if required—in         particular, into panels—and in that the thus cut metallic layer         is brought into contact with at least one layer on the basis of         organic polymer and is combined into a sandwich structure by         means of compaction under pressure and/or temperature.

Preferably, the organic polymer is a thermoplastic polymer and/or a thermoplastic copolymer.

Before the coating of surfaces of the metallic layers, the surfaces to be coated are typically cleaned and then rinsed with water. If metallic layers are put into intermediate storage, they are usually oiled. If oiled surfaces are to be treated or if the plant is at least somewhat contaminated with oil, it is to be recommended particularly to clean the surfaces with an alkaline cleaner before the coating and then to rinse them with water in order to remove adhering cleaning solution from the surface. A surface that is not completely water-wettable can result in an uneven no-rinse coating.

Before the application of the inventive aqueous conversion composition, no activation is used, since it is desirable to generate an extremely fine-grained and, at the same time, largely or completely amorphous phosphate coating and as far as possible no crystalline phosphate coating. The resulting conversion layer is preferably completely X-ray amorphous or is X-ray amorphous with slight crystalline fractions, which is unusual for a zinc phosphate layer. It is important that the surface of the metallic layer is wetted as completely and evenly as possible with the no rinse conversion composition. On application with the aid of rollers as coater rollers and/or as squeezing rollers, the condition of the application roller is very important since it is intended to create an even wet film preferably in the range from 0.5 to 10 ml/m².

The inventive conversion coating method therefore takes place without an activation step with a colloidal titanium phosphate or with a surface conditioner on the basis of phosphate particles, which is unusual for a typical zinc phosphating. This is also a sign that the inventive conversion composition and its coating method are no typical zinc phosphating.

Preferably, the aqueous conversion composition has a total content of manganese, aluminum, chromium(III), iron(II), iron(III) and/or magnesium in the region of 0.01 to 8 g/l, particularly preferably in the range from 0.1 to 6 g/l, from 0.2 to 5 g/l, from 0.3 to 4 g/l, from 0.4 to 3 g/l, from 0.5 to 2 g/l or from 0.6 to 1 g/l.

Preferably, the aqueous conversion composition has the following composition:

-   -   1 to 10 g/l zinc,     -   0.5 to 6 g/l manganese, 0.01 to 0.5 g/l aluminum, 0.01 to 0.8         g/l chromium(III), 0.01 to 1 g/l iron(II), 0.01 to 1 g/l         iron(III) and/or 0.01 to 1.5 g/l magnesium, wherein the total of         these elements/cations without zinc preferably lies in the range         from 0.01 to 8 g/l,     -   0 or 0.01 to 2.5 g/l nickel, 0 or 0.01 to 2.5 g/l cobalt,         wherein the total of nickel and cobalt is 0 or lies in the range         from 0.01 to 4 g/l,     -   0 or 0.01 to 5 g/l of the total of molybdenum and/or vanadium,     -   8 to 60 g/l P₂O₅, which corresponds to 10.72 to 80.28 g/l PO₄,     -   0.5 to 5 g/l polyacrylic acid, but not more than 25% of the P₂O₅         content of the composition in g/l, and     -   0 or 0.01 to 3 g/l silane, but not more than 25% of the P₂O₅         content of the composition in g/l, and also no content of a         complex fluoride of titanium or zirconium.

Particularly preferably, the aqueous conversion composition has the following composition:

-   -   2 to 8 g/l zinc,     -   1 to 5 g/l manganese, 0.01 to 0.2 g/l aluminum, 0.01 to 0.5 g/l         chromium(III), 0.01 to 0.5 g/l iron(II), 0.01 to 0.5 g/l         iron(III) and/or 0.01 to 0.8 g/l magnesium, wherein the total of         these elements/cations without zinc preferably lies in the range         from 0.01 to 8 g/l,     -   0 or 0.01 to 2 g/l nickel,     -   0 or 0.01 to 2 g/l cobalt, wherein the total of nickel and         cobalt is not more than 2.5 g/l,     -   0 or 0.01 to 5 g/l of the total of molybdenum and/or vanadium,     -   9.5 to 50 g/l P₂O₅, which corresponds to 12.73 to 66.9 g/l PO₄,     -   0.5 to 3 g/l polyacrylic acid, but not more than 25% of the P₂O₅         content of the composition in g/l, and     -   0 or 0.01 to 2 g/l silane, but not more than 25% of the P₂O₅         content of the composition in g/l, and also no content of a         complex fluoride of titanium or zirconium.

Preferable herein is an initial aqueous conversion composition in which the ratio by weight of zinc:PO₄ lies in the range from 0.02:1 to 0.6:1, from 0.05:1 to 0.5:1, from 0.08:1 to 0.4:1 or from 0.1:1 to 0.2:1 relative to the initial composition before an uptake of steeped zinc which can become enriched in the acidic solution over time.

Preferable herein is an aqueous conversion composition in which the ratio by weight of zinc:PO₄ lies in the range from 0.02:1 to 8:1, from 0.05:1 to 4:1, from 0.08:1 to 1.5:1 or from 0.1:1 to 0.8:1, related to the composition which has additionally already taken up etched out zinc when used, for example, on galvanized surfaces.

The ratio by weight of the aqueous conversion composition and/or the coating produced therefrom of the total (Ni and Co):Zn preferably lies in the range from 0.0001:1 to 1:1, from 0.002:1 to 0.7:1, from 0.01:1 to 0.4:1 or from 0.04:1 to 0.2:1. Depending on the content of nickel and/or of cobalt, the corrosion-resistance of the conversion coating created therewith can be influenced.

Preferably, the zinc content of the aqueous conversion composition is 1.5 to 10 g/l, 2 to 8 g/l or 2.5 to 4.5 g/l. Preferably, the manganese content of the aqueous conversion composition is 0.5 to 6 or 1 to 5 g/l. Preferably, the nickel content of the aqueous conversion composition is not more than 1.5 g/l, not more than 1 g/l, not more than 0.5 g/l or not more than 0.01 g/l. The Zn:Mn ratio by weight of the aqueous conversion composition can vary within wide limits and preferably lies in the range from 0.1:1 to 5:1 or from 0.8:1 to 3:1.

The aqueous conversion composition can possibly also contain 0.001 to 1 g/l NO₂, 0.01 to 1 g/l NO₃ and/or 0.001 to 2 g/l H₂O₂ as accelerant.

An addition of polyacrylic acid to the aqueous conversion composition has proved to be useful since therewith significantly better adhesion results are achieved, which had not previously succeeded without polyacrylic acid. An addition of polyacrylic acid provides the conversion coating with a greater adhesive strength, which has a positive effect, in particular with materials on the basis of polyethylene and/or polyamide. It is herein advisable to add a water-soluble or water-dispersible polyacrylic acid to the conversion composition. The polyacrylic acid preferably has a molecular weight in the range from 5,000 to 120,000, from 30,000 to 90,000 or from 50,000 to 70,000. The conversion composition preferably has a content of at least one water-soluble or water-dispersible polyacrylic acid in the region of 0.2 to 8 g/l, from 0.3 to 5 g/l or from 0.5 to 3 g/l. The addition of polyacrylic acid has had positive effects with all the different metallic surfaces, on the adhesion firstly to the metallic surface, and secondly to the surface wetted with organic, particularly thermoplastic, plastics. Preferably, the content of polyacrylic acid and/or its reaction products in the dried-on and/or dried-on and, during compaction, thermally loaded conversion coating is 0.05 to 15% by weight of the conversion coating.

If additionally at least one silane is added to the conversion composition, the adhesion between adjacent layers of a sandwich structure, measured here as the peel strength, can be further improved by up to approximately 25%.

If the content in the aqueous conversion composition of polyacrylic acid is successively increased and finally exceeds approximately 25% of the P₂O₅ content in g/l, it can arise that no further increase in the peel strength of the sandwich structure than at lower content levels, or even a somewhat lower peel strength, is achieved.

If the content of polyacrylic acid in the aqueous conversion composition falls below approximately 2% of the P₂O₅ content in g/l, it can occur that a significant worsening in the peel strength of the sandwich structure as compared with higher levels of polyacrylic acid, results.

For the sake of simplicity, silane, silanol and siloxane are often referred to below simply as silane. The reason is that silane/silanol/siloxane if often also subject to a rapid sequence of reactions, including in water, so that due to the changes and due to the great effort for a suitable demonstration of the state, a determination and a more precise specification are not worthwhile.

As silanes, fundamentally many types are possible individually or in combination with one another. They are preferably put into a water-soluble or water-dissolved state. Particularly preferably, they are partially or entirely hydrolyzed.

Particularly preferable are conversion compositions with a content of at least one aminosilane with at least one amino group and of at least one aminosilane different therefrom with at least two amino groups, at least one aminosilane, and at least one epoxysilane with a content of at least one aminosilane and at least one bis-silyl-silane with a content of at least one ureidosilane and at least one bis-silyl-silane or with a content of at least one epoxysilane and at least one bis-silyl-silane.

Particularly preferred are silanes with respectively at least one amino, epoxy, glycidoxy, imino and/or ureido group. Particularly preferred are mono-trialkoxysilanes, bis-trialkoxysilanes, mono-silyl-silanes and/or bis-silyl-silanes, wherein these particularly preferably have at least one amino, epoxy, glycidoxy, imino and/or ureido group.

The conversion composition advantageously has a total content of at least one silane in the range of 0.01 to 4, from 0.1 to 3, 0.3 to 2.5 or from 0.6 to 2 g/l.

If the content of silane(s) in g/l in the aqueous conversion composition exceeds approximately 25% of the content of P₂O₅ in g/l, it can occur that the adhesion worsens. If the content of silane(s) in g/l in the aqueous conversion composition falls below approximately 2% of the content of P₂O₅ in g/l, it can occur that the adhesion worsens.

If the polyacrylic acid content in g/l in the aqueous conversion composition exceeds approximately 25% of the of P₂O₅ content in g/l, it can occur that the adhesion worsens.

If the of polyacrylic acid content in g/l in the aqueous conversion composition falls below approximately 2% of the P₂O₅ content in g/l, it can occur that the adhesion worsens.

Preferably, the content of at least one silane and/or its/their reaction products in the dried-on and/or dried-on and, during compaction, thermally loaded, conversion coating is 0.01 to 15% by weight of the conversion coating.

However, in some experiments, it has been found that the polyacrylic acid content in the aqueous conversion composition should preferably be not more than 25, not more than 20, not more than 15 or not more than 10% of the P₂O₅ content in g/l of the composition. Under some circumstances, no further improvement of the peel strength can be achieved as compared with compositions having less polyacrylic acid.

However, in some experiments, it has been found that the content of at least one silane in the aqueous conversion composition should preferably be not more than 25, not more than 20, not more than 15 or not more than 10% of the P₂O₅ content in g/l of the composition. Under some circumstances, no further improvements of the peel strength can be achieved as compared with compositions having less silane.

The aqueous conversion composition can preferably also contain 0.001 to 20 g/l or 0.2 to 10 g/l of at least one further water-soluble and/or water-dispersible organic polymer/copolymer apart from polyacrylic acid.

If, furthermore, at least one further organic polymer/copolymer is added to the aqueous conversion composition, this is preferably selected from acid-resistant polymers and/or copolymers, which are specifically stabilized, if relevant. These include, in particular, acid-resistant polymers and/or copolymers selected from those on the basis of poly(meth)acrylate, polyacrylamide, polycarbonate, polyepoxide, polyester, polyether, polyethylene, polystyrene, polyurethane, polyvinyl, polyvinylpyrrolidone and modification(s) thereof, for example cationic polyurethane resin, modified anionic acrylate/polyacrylate, epoxy resin with amino groups and, if relevant, also with phosphate groups and/or cationic copolymer on the basis of polyester-polyurethane, polyester-polyurethane-poly(meth)acrylate, polycarbonate-polyurethane and/or polycarbonate-polyurethane-poly(meth)acrylate.

It can herein be advantageous if the conversion composition also has a content of at least one further organic polymer/copolymer and at least one silane/silanol/siloxane. These additives can contribute, for example, to increasing the adhesive strength.

Preferably the aqueous conversion composition and, if relevant, also the corresponding conversion coating has, in many embodiments, no content of boron, chromium(VI), hafnium, titanium, zirconium and/or niobium or no content of an intentionally added compound of boron, chromium(VI), hafnium, titanium, zirconium, niobium or of aluminum, boron, chromium(VI), hafnium, titanium, zirconium, niobium, tantalum, vanadium and/or tungsten or no content of an intentionally added compound of boron, chromium(VI), hafnium, titanium, zirconium, niobium, tantalum and/or tungsten. In particular, it is preferred that it has no content of a complex fluoride, in particular of aluminum, boron, silicon, hafnium, titanium and/or zirconium. In many variants, the inventive aqueous conversion composition has no content of other organic polymers and copolymers except from polyacrylic acid, a cross-linking agent, a photoinitiator and/or a chromium(III) compound. Preferably, the inventive conversion composition has no content of metallic and/or inorganic particles, in particular such particles larger than 0.1 μm.

In the conversion coating method, the pH value of the conversion composition can lie in the range from 1 to 5, preferably in the range from 2.2 to 4.5, particularly preferably in the range from 2.8 to 4.

Particularly preferably, in the inventive method, as the inorganic aqueous basic composition, one such is selected which enables a “microphosphating” to be carried out with the aqueous conversion composition. Microphosphating is herein referred to when the layer weight of the conversion coating is less than or equal to 0.4 g/m² or less than or equal to 0.3 g/m². Therefore, in the inventive method, a conversion coating with a layer weight of up to 0.4 g/m² is preferably formed. Herein, the zinc phosphate is preferably present at least as partially X-ray amorphous, since no typical zinc phosphating is carried out.

The inventive phosphate-rich conversion coating can be largely or entirely amorphously configured in many embodiments, and far finer than a typical finely crystalline zinc phosphate layer. In the inventive conversion coating, neither particles nor the finest hollow spaces can be discerned with the naked eye, so that this coating makes a far more even and unified impression compared with a typical “normal” zinc phosphate layer. This often succeeds only if no activation and no surface conditioning has been used before the conversion coating, if the wet film of the aqueous conversion composition has been homogeneously formed on the surface of the metallic layer and if the contact times with the aqueous conversion composition until complete drying out were comparatively short, in particular, shorter than 1 minute. The activation or surface conditioning before a phosphate-rich conversion composition serves to coat the surfaces to be coated with seed crystals for forming phosphate crystals.

A typical zinc phosphating is introduced with an activation or surface conditioning of the metallic surface and causes a longer contact time of the zinc phosphate solution with the metallic surface, specifically typically depending on the application type by spraying and/or dipping and depending on the substrate type as a band or as parts, of between 3 s and approximately 5 min. Herein, typically bath temperatures in the range from 50 to 70° C. and substrate temperatures in the range from 15 to 40° C. are used. For typical zinc phosphating, squeezing is not used except with band. For typical zinc phosphating, drying out is not used, although rinsing is. Due to all these measures, the phosphate layer of a typical zinc phosphating is unusually formed in crystals of approximately 5 to 40 μm. Their layer weight is herein typically in the range from 1 to 25 g/m².

In the inventive conversion coating method, it is preferred to bring the inventive aqueous conversion composition into contact with the metallic surface, in particular, of a band for 0.1 to 30 s until water-free during drying-on, wherein the temperatures of the metallic surface and of the band preferably lie in the region of 15 to 40° C. Due to the omission of an activation or surface conditioning and due to the shorter contact times and the possibly also lower temperatures, the crystallization of the zinc phosphate is largely or entirely suppressed so that it is predominantly or entirely present amorphously and much finer than in a crystalline zinc phosphate layer. These properties also characterize the microphosphating.

Furthermore, it is preferred in many embodiments that the liquid film applied is dried on without being rinsed therein or thereafter with aqueous liquid. Alternatively, rinsing can be performed with an aqueous solution, for example, of a salt and/or another compound, for example, in order to improve the adhesion still further.

In the inventive conversion coating method, the surfaces of the metallic layer can preferably be coated at a peak-metal temperature, PMT, in the range from 5 to 50° C., particularly at 15 to 40° C. The aqueous conversion composition can have a temperature at the time point of the application in the range from 15 to 50° C. or from 20 to 40° C., wherein a t temperatures of above 40° C., it must be noted whether precipitations possibly arise in the conversion composition.

In the inventive conversion coating method, the contact times until water-freedom when drying onto surfaces of the metallic layer, particularly in the case of metallic band or metallic coil are, in particular, 0.2 to 30 seconds. In the case of rolling and/or spraying onto band, the time of contacting until water-freedom can, under some circumstances, be reduced to less than 1 second or to approximately 0.2 seconds. In particular, on a conversion coated metallic band or coil, the drying can take place, for example, in a heated air stream, by induction and/or by IR and/or NIR heat radiation. The conversion coating of the metallic layer takes place, above all, before the bringing together of this layer with at least one polymer layer for compressing to a sandwich structure.

The polymer layers can have thickness, for example, in the range from 0.1 to 5 mm, from 0.2 to 4 mm, from 0.4 to 3 mm, from 0.6 to 2 mm, from 0.7 to 1.2 mm or of approximately 0.8 mm per length. They preferably consist of compact plastics and not of plastics foam.

For example, a plastics coil can be used as the polymer layer. In moisture-sensitive plastics, this is possibly contained vacuum-welded into film. This film is to be removed from the polymer layer for compression. Many polypropylene-based plastics have no moisture-sensitivity. Many types of polymer layer and also the metallic layers are usually not to be further pre-treated. The polymer layers and the metallic layers can often be compressed after the bringing together and placing on one another in advantageous manner without prior heating.

Two embodiments as to how lamination layers can be built up and how they can function are described, by way of example, in WO 2009/043777 A2.

If a number of layers of metal and plastics are used for the production of a sandwich structure, it is preferred that, firstly, at least two, at least three or at least four metallic layers of metal and/or metallic alloy and/or metallic layers with at least one additional metal or alloy layer, for example, galvanized steel and/or, secondly, at least two or at least three polymer layers (=polymer layers), which can possibly also be coated on at least one side and/or specially treated, for example, polarized by flame-scorching, UV treatment, corona treatment or plasma treatment, are used. Herein, it is particularly preferable that each metallic layer alternates with a polymer layer.

Herein, for example, the following sequences of layers can occur, wherein if two polymer layers adjoin one another, both similar and different plastics or polymer layers can be used with similar or different dimensions and/or properties independently of one another: metal-plastics, metal-plastics-metal, metal-plastics-plastics, metal-plastics-plastics-metal, metal-plastics-plastics-plastics-metal, metal-plastics-metal-plastics-plastics, plastics-plastics-metal-plastics, plastics-plastics-metal-plastics-plastics, metal-plastics-plastics-metal-plastics, metal-plastics-plastics-metal-plastics-plastics, plastics-metal-plastics-plastics-metal-plastics, plastics-metal-plastics-plastics-metal-plastics-plastics, plastics-plastics-metal-plastics-plastics-metal-plastics-plastics, metal-plastics-plastics-metal-plastics-metal or metal-plastics-plastics-metal-plastics-plastics-metal.

Herein, more complex structures, structures with inlays and/or structures without continuous layers and/or with hollow spaces are also producible, wherein these peculiarities can each relate to one or more metallic layers and/or respectively one or more polymer layers. It is particularly preferred, however, that the polymer layers and/or the metallic layers have no hollow spaces and/or as far as possible no pores.

For the production of sandwich structures of one or more metallic layers, for example, of metallic coil or sheet metal sections and one or more polymer layers of organic, in particular thermoplastic, polymer, for example, plastics coil or polymer sections, gluing and/or compaction under pressure and/or temperature are particularly suitable as joining methods.

Many types of organic polymers are suitable as polymer layers, in particular also as polymer core layers between metallic layers, wherein in particular, temperature-resistant and/or mechanically loadable layers are preferred. Polymer layers of an organic composite material (=compound) with proportions of different plastics are herein preferred. In particular, layers of thermoplastics, plastics with a proportion of thermoplastics and/or fiber-reinforced thermoplastics can be used as layers of organic polymer. If at least three polymer layers are used in contact with one another, the at least one middle layer can alternatively also consist of a plastics with a proportion of thermoplastics of less than 50% by weight or even of thermosetting plastics, possibly respectively also fiber-reinforced.

Plastics or thermoplastics that are suitable are, in particular, those on the basis of polyamide, polyethylene and/or polypropylene, which can possibly be fiber-reinforced, in particular with aramide, glass, carbon and/or graphite fibers. Preferably, the layer of organic polymer is a layer on the basis of at least one thermoplastics and, in particular, on the basis of plastics on the basis of polyamide, polyethylene and/or polypropylene, which can possibly also be fiber-reinforced. Particularly preferred are possibly fiber-reinforced thermoplastics on the basis of polyamide, polyamide and polyethylene or polyamide and polypropylene. In particular, polymer layers of a composite material (compound) with a proportion of PA 6 and/or PE can be used, which means a polyamide on the basis of ε-caprolactam or ω-aminohexanoic acid and/or of polyethylene, which can possibly also be fiber-reinforced. Particularly preferred is an organic composite material of PA 6 with a proportion of PE, that is, a polyamide on the basis of ε-caprolactam and/or ω-aminohexanoic acid and of polyethylene.

These organic materials are particularly preferred since they are temperature-resistant and/or mechanically loadable. These composite materials can preferably contain additives, for example, stabilizers, adhesion-promoters and/or compounds with adhesive groups. These organic composite materials can preferably be made so that the organic composite materials or layers of these organic composite materials are preferably entirely pore-free and therefore are not foam and/or are deep-drawing capable at a temperature of up to approximately 100° C. and/or are ductile at room temperature or possibly up to 220° C.

Herein, for large-scale manufacturing, it suggests itself to use band as the metallic starting material, particularly as a coil or as cut sheet, which is or has been coated in a coil coating process and which is joined to at least one layer of a plastics material.

Particularly preferred are metallic substrates as coil, band and/or sheet, which often have a thickness of the layer in the range, in particular, from 0.1 to 2 mm and, if required, can be coated with a metallic protective layer and/or with at least one protective layer, for example, on the basis of an adhesion promoter, passivator or pre-treatment layer. A good compromise between the weight and properties of the sandwich structure has proved to be the use of a band/coil/sheet with a thickness in the range from 0.05 to 1 or from 0.1 to 0.5 mm, preferably from 0.2 to 0.3 mm. Metallic coatings which are used in place of a metallic layer or on a metallic layer can herein have a thickness in the range from 1 to 80 μm.

As metallic surfaces, in particular, those of aluminum, iron, magnesium, titanium, zinc, tin and alloys thereof come into question. As metallic substrates, in particular, those based on aluminum, iron/steel/high-grade steel, magnesium, titanium, zinc, tin and alloys thereof can be used. As the coating of a metallic layer, for example metal such as e.g. zinc, an alloy containing aluminum, magnesium and/or zinc, for example, a ZnMg alloy or a chrome plating, in particular, on steel can be used.

As metallic bands, coils and/or sheets and in particular for use as cover sheets, above all metallic layers can be used which possibly can be coated with a metallic layer and/or with at least one protective layer, for example, on the basis of an adhesion-promoter, passivation or pre-treatment layer and which are made of one of the following materials: electrolytically galvanized steel sheet ZE, for example, electrolytically with a magnesium and/or zinc-containing alloy, for example, steel sheet coated with ZnNi, ZnCo, ZnMg, hot-dip galvanized steel sheet Z, by hot-dipping in a molten aluminum-containing and/or zinc-containing alloy, for example, a steel sheet coated with an Al, ZnAl, ZnMg and/or ZnAlMg alloy, chrome-plated sheet, for example, chrome-plated steel sheet, sheet metal made of aluminum, aluminum alloy, magnesium, magnesium alloy and/or high-grade steel. If a metallic layer is coated, it can be coated on one side or both sides. These coatings, depending on the side, can be different or the same in their thickness and/or characteristics and/or composition and can additionally have, for example, at least one conversion layer and/or a passivation layer on a metallic coating.

Layers of different materials can also be combined with one another, for example, an upper layer of a high-grade steel sheet with a lower layer of galvanized steel sheet or, for example, in a combination of metal-plastics-metal-plastics-metal, on the outside, high-grade steel sheets and as at least one middle layer, a galvanized steel sheet and/or two different plastics types and/or polymer layers with different properties.

The inventive method is particularly advantageous if at this point in the method, following the bringing together and laying on one another of at least one metallic layer with at least one polymer layer for compression to a sandwich structure, only the compression is still required. Preferably, panels or coils are used as polymer layers and as metallic layers for compression, which influences the type, design and size of the plant. If coils of plastics and of metallic material are used, the compression can preferably take place continuously and on compression of panels, often discontinuously, for example, in a double band press. Particularly in continuous methods, pre-heating of the individual or combined layers can possibly be helpful. For a continuous compression, preferably, a plant can be used in which the pressing plates of the pressing tool remain static for the pressing time or, if the transport speed is low, move along with the band plant.

The compression of the metallic layers and polymer layers placed against one another preferably takes place at temperatures of the pressing tool and particularly of the pressing plates in the range from 100 to 260° C., in particular from 120 to 240° C. It is herein preferred that the temperature of the pressing plates is kept as constant as possible. In a particularly preferred method variant, during compression, the heated pressing plates which are configured planar and arranged mutually parallel can be pressed for a particular duration onto the planar structure of the layers lying on one another, without the pressing plates being lifted. The pressing duration is preferably 5 s to 20 min, 10 s to 8 min or 30 s to 8 min, wherein on compression of overall more than two layers, a longer pressing duration and/or a slightly raised temperature is/are rather used. During compression, a sufficiently long action of the heat at or above the melting temperature of at least one organic, in particular, thermoplastic component of a compound or of the, in particular, thermoplastic plastics of the polymer layer is to be ensured.

Preferably, the melt temperature of at least one component of the compound or of the, in particular, thermoplastic plastics of the polymer layer is reached or exceeded over at least 5 s and/or up to 10 min. The melt temperature of at least one component of the compound or of the plastics of the polymer layer often lies in the range from 150 to 260° C. or from 2 00 to 250° C. The plastics of the polymer layer herein melts at least partially, attaches firmly to the conversion-coated surface of the metallic layer and becomes adhesively connected thereto. The pressing pressure during compression is preferably in the range from 0 to 10 bar or from 2 to 8 bar. In the inventive method, it is necessary that the at least one metallic layer has a coating of the inventive conversion coating on the surface that is to be joined to plastics, said coating acting there as an adhesion promoter and as a corrosion protection at the future cut edges. This is the case because it has been found that the edge protection is generally better if the adhesion of the inventive coating is increased.

If the pressing plates are lifted at the end of the compression process, the sandwich structure can be cooled by natural cooling and/or by forced cooling, for example, with cool air. Until the beginning of the winding up of a laminated-together coil or until the stapling of laminated-together panels, the temperature should typically be below 90° C.

When the metallic coils (bands) are coated with the inventive conversion composition and dried they can be cut, if necessary, into panels which can be compressed with the polymer layers at a later time point, or the coils (bands) are continuously compressed—in particular, in a laminating plant on passing through with the polymer layer. It should herein be ensured that until the joining with the polymer layer, the conversion coating is not chemically or mechanically loaded, so as not to damage the conversion coating therethrough. The inventive conversion coating on the surface of the metallic layer which is preferably directly joined to the polymer layer, provides the necessary adhesion for a lasting bond between the solidified polymer layer and the metallic layer.

In particular when such sandwich structures are further coated as components or used as one of a plurality of components and coated thereafter, a coating with respectively at least a liquid paint, an electrocoat paint, for example, a cathodic KTL electrocoating and/or a powder coating is fundamentally possible. Coating with an electrocoat and/or a powder coating is particularly advantageous herein since even hard-to-access and hidden sites can be coated. Particularly preferred are polymer layers which can be used as temperature-resistant polymer layers or as temperature-resistant polymer core layers. Electrocoatings of this type are often applied at temperatures in the range from 15 to 40° C. and mostly at temperatures in the range from 25 to 30° C. under the action of current. These electrocoatings are subsequently baked in an oven, often at temperatures of approximately 160 to 180° C. Furthermore, within the most varied of paint coatings, paints such as for example a finishing paint, a filler, a clear lacquer or a powder coating are used which can be baked at temperatures approximately in the range from 150 to 240° C. Thus, the expression “temperature-resistant” for a polymer layer of the present invention means that it withstands the baking temperatures in the baking oven without any disadvantageous influence on their properties. Therefore, overall, at least one electrocoating, finishing paint, filler, clear lacquer and/or powder coating can be applied onto at least one side of the composite or laminate.

The inventive sandwich structure can, if required, be at least once respectively coated, deformed, glued, compressed and/or otherwise joined. From sandwich structures of this type, for example, bodywork parts can be produced, for example, by cutting, deforming, painting and/or joining, wherein the sequence of these steps can be varied.

Unexpected Effects:

It was surprising that through the addition of polyacrylic acid in the range from 1 to 3 g/l, significantly greater peel strength values could be achieved than with the previously tested and/or used conversion compositions.

It was also surprising that the conversion composition generated a sufficient adhesion between the metallic surface and the polymer layer on all such different metallic substrates such as, for example, CRS, Z, ZE, ZnMg, ZnAlMg, high-grade steel and Al.

It was further surprising that the conversion coating also did not disrupt a subsequent cathodic electrocoating (KETL), even on the side of the metal sheet not joined to a polymer layer.

It was advantageous that two successively performed conversion coatings could be reduced to a single coating, such that two to four process steps can be spared. Apart from this simplification of the production process, a significant increase in the peel strength could also be achieved.

The inventive production and the inventive sandwich structures can be used in motor vehicle construction, aircraft construction, space travel, apparatus construction, in building construction, furniture production or as structural elements.

The inventive sandwich structures can be used, in particular, as parts of motor vehicles, trailers, mobile homes or aerodynamic vehicles, as parts of bodywork, as elements of doors, rear hatches or engine hoods, as fenders, as crash barriers, for electrotechnical equipment, for domestic appliances, external claddings, facade elements, roof claddings, garage door elements, fence elements, in interior fittings, as radiators, as lamps, for furniture items or as furniture elements, for wardrobe elements or shelving, as bands, as metal sheets, formed parts, coverings, paneling, screens, frames, isolating elements, safety elements, supports or as housings.

EXAMPLES AND COMPARATIVE EXAMPLES

The subject matter of the invention will now be described in greater detail by reference to exemplary embodiments:

The examples below are based on the following substrates or method steps:

The following sheet metals were tested:

A) Aluminum alloy AlMg3 5754 W19, B) Cold rolled continuous annealed steel sheet (CRS) from unalloyed steel DC04B, C) Light gage electrolytically galvanized sheet steel (ZE) in automobile quality, grade DX54 DZ100, D) hot-dip galvanized rerolled sheet (Z) from mild unalloyed steel, grade DX53 with at least 100 g/m² zinc deposit, E) High-grade steel (StS), grade 1.4301, F) Magnesium alloy AZ31, and G) Magnesium alloy AM50, each with a thickness of approximately 0.2 to 1 mm, depending on the metallic material and test. 1. The substrate surfaces of the sheets were cleaned in a 2% aqueous solution of a strongly alkaline cleaner over 10 to 20 s at 55 to 60° C. and thoroughly degreased in the process. 2. This was followed by rinsing with mains water for 0.5 minutes at room temperature. 3. Different aqueous conversion compositions were prepared according to Table 1, all except VB37 having good bath stability. The salts used herein are given on the second and third pages of the tables, although always only one of a plurality of the zinc compounds given was added. As polyacrylic acid, an adhesion-promoting formulation dissolved in water with polyacrylic acid polymer having a mean molecular weight in the range from 50,000 to 70,000 was used. As epoxysilane 1, 3-glycidoxypropyltrimethoxysilane in the pre-hydrolyzed state was used. As aminosilane 1, N-(2-(aminoethyl)-3-aminopropyltrimethoxysilane in the prehydrolyzed state was used. The conversion compositions showed a pH value in the range from 2 to 3. 4. In many tests, a plurality of different metallic substrates were treated one after another and otherwise treated in the same manner and grouped together under a number of an example or comparative example for greater clarity in Table 1. 5. Thereafter, the surfaces of the different types of metal sheet given in Table 1 under substrates were coated at room temperature with a laboratory coater, wherein a wet film of approximately 3.5 ml/m² was applied. 6. Then the coated substrates were dried in a drying oven at 180° C. for 20 to 30 s at a peak metal temperature PMT of 60° C., where in the wet film was dried on without prior rinsing. 7. On the dried conversion-coated substrates, the layer weight for the chemical element phosphorus was determined with an X-ray fluorescence analysis apparatus (RFA) to discover the P₂O₅ content. In examples B1 to B6, layer weights for the conversion coating in the range from 20 to 60 mg/² P₂O₅ were tested, whereas in the further examples and comparative examples, a layer weight in the range from 10 to 150 mg/m² P₂O₅ was used. Table 1 shows extracts therefrom. 8. As polymer layers, KTL temperature-resistant layers made of a composite material (compound) of polyamide PA6 with a proportion of PE, which means a polyamide on the basis of ε-caprolactam or ω-aminohexanoic acid and/or of polyethylene having a thickness in the range from 0.05 to 1.00 mm. 9. In each case, a polymer layer was pressed in a panel press as an intermediate layer between two identical metallic layers conversion-coated according to the invention without targeted pre-warming, under loading at a temperature possibly of up to 240° C. without pressure for 60 s and thereby combined to a durable sandwich structure. Herein, at least a part of the, in particular, thermoplastic material melted and, on cooling, formed a firm bond by means of the inventive conversion coating. 10. Following cooling, peel strength values to ISO 11339:2010 were determined, wherein triple measurements per sandwich structure were made on relevant samples. The portions used from the sandwich structure were 4 cm wide and 13 cm long and were expected to produce peel strength results of at least 300N/4 cm, i.e. at least 75N/cm in the peel strength test to ISO 11339:2010.

TABLE 1 Composition and properties of the aqueous conversion compositions, the conversion coatings and the sandwich structures produced therewith. B1 B2 B3 B4 B5 B6 B7 B8 B9 B10 B11 B12 Substrate Z/ZE CRS/StS Z/ZE/CRS/AL/StS Z/ZE Bath composition in g/l; remainder: water Zn  3.33 3.33 3.33 6.86 6.86 6.86 8.76 8.76  8.76  1.72 1.72  1.72 Mn  2.02 2.02 2.02 4.03 4.03 4.03 5.15 5.15  5.15  1.01 1.01  1.01 Ni  0.69 0.69 0.69 1.37 1.37 1.37 1.75 1.75  1.75  0.34 0.34  0.34 Mo  0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10  0.10  0.10 0.10  0.10 P₂O₅ 19.15 19.15 19.15 38.30 38.30 38.30 48.86 48.86 48.86  9.57 9.57  9.57 PO₄ 25.62 25.62 25.62 51.24 51.24 51.24 65.39 65.39 65.39 12.81 12.81 12.81 Zn:PO₄  0.13 0.13 0.13 0.13 0.13 0.13 0.13 0.13  0.13  0.13 0.13  0.13 Polyacrylic acid  1.28 2.56 1.59 1.28 2.56 1.59 1.28 2.56  1.59  1.28 2.56  1.59 Epoxysilane 1 1.56 1.56  1.56  1.56 Layer weight P₂O₅ 20-60 40-120 50-150 10-30 mg/m² Peel strength N/4 cm 500- 500- 400- 300- 300- 350- 300- 300- 350- 200- 200- 400- 700 750 500 400 400 650 400 400 650 350 350 500 B13 B14 B15 B16 B17 B18 B19 B20 B21 B22 B23 Substrate CRS/StS Z/ZE CRS/StS Z/ZE CRS/StS added as Bath composition in g/l; remainder: water Zn as ZnO + ZnCO₃  3.33 3.33 3.33 3.33 6.86 3.33 3.33 3.33  6.86 6.86  6.86 Mn as Mn₃(PO₄)₂  2.02 2.02 2.02 2.02 4.03 2.02 2.02 2.02  4.03 4.03  4.03 Ni as NiCO₃  0.69 0.69 0.69 0.69 0.69 0.69  1.37 1.37  1.37 Al as AlPO₄ 0.18 0.18 0.18     Cr(III) as Cr(NO₃)₃  0.2 0.4  0.2 Mo  0.10 0.10 0.10 0.10 0.10 [(NH₄)₆Mo₇O₂₄₋4H₂O] NO₃ as NaNO₃ 0.60 0.60 P₂O₅ 19.15 19.15 19.15 19.15 38.30 19.15 19.15 19.15 38.30 38.30 38.30 PO₄ 25.62 25.62 25.62 25.62 51.24 25.62 25.62 25.62 51.24 51.24 51.24 Zn:PO₄  0.13 0.13 0.13 0.13 0.13 0.13 0.13 0.13  0.13 0.13  0.13 Polyacrylic acid  1.28 2.56 1.59 2.56 2.56 1.28 2.56 1.59  1.28 2.56  1.59 Epoxysilane 1 1.56 1.56  1.56 Aminosilane 1  0.20 Layer weight P₂O₅ 20-60 20-60 20-60 20-60 40-120 mg/m² Peel strength N/4 cm 400- 400- 400- 500-750 300-400 500-750 — 400-500 — — — 500 500 500 B24 B25 B26 B27 B28 B29 B30 B31 B32 B33 B34 Substrate Z/ZE/CRS/AL/StS Z/ZE CRS/StS Z/ZE added as Bath composition in g/l; remainder: water Zn as ZnO + ZnCO₃  8.76 8.76 8.76 1.72 1.72 1.72 3.33 3.33  3.33 3.33  6.86 Mn as Mn₃(PO₄)₂  5.15 5.15 5.15 1.01 1.01 1.01 2.02 2.02  2.02 2.02  4.03 Ni as NiCO₃  1.75 1.75 1.75 0.34 0.69 0.69  0.69 Co as Co(NO₃)₂₋4H₂O 0.34 0.34 Fe(III)[Fe(NO₃)_(2.)9H2O] 0.20  0.20  0.40 Mg as Mg₃(PO₄)₂ 0.30 0.60 Mo  0.10 0.10 [(NH₄)₆Mo₇O₂₄₋4H₂O] V as NaVO3-W 0.20 W 0.10 V as Na₂WO₄ H₂O₂  0.0002  0.0004 NO₂ as NaNO₂ 0.002 NO₃ as NaNO₃ 0.60  0.60  0.60 P₂O₅ 48.86 48.86 48.86 9.57 9.57 9.57 19.15 19.15 19.15 19.15 38.30 PO₄ 65.39 65.39 65.39 12.81 12.81 12.81 25.62 25.62 25.62 25.62 51.24 Zn:PO₄  0.13 0.13 0.13 0.13 0.13 0.13 0.13 0.13  0.13 0.13  0.13 Polyacrylic acid  1.28 2.56 1.59 1.28 2.56 1.59 1.28 2.56  1.59 2.56  2.56 Epoxysilane 1 1.56 1.56  1.56 Layer weight P₂O₅ 50-150 10-30 20-60 40-120 mg/m² Peel strength N/4 cm 300- — 350- 200- 200- 400- 200- 200- 200- 300- 300- 400 650 350 350 500 320 320 320 420 400 B35 B36 VB37 B38 B39 B40 B41 B42 B43 VB44 VB45 VB46 B47 VB48 VB49 Z/ZE Substrate Bath composition in g/l; remainder: water Zn  1.28 6.41 12.81 3.33 3.33 3.33 3.33 3.33  3.33  3.33  3.33  3.33  3.33  3.33  3.33 Mn  2.02 2.02 2.02 2.02 2.02 2.02 2.02 2.02  2.02  2.02  2.02  2.02  2.02  2.02  2.02 Mo  0.10 0.10 0.10 0.10 0.10 0.10  0.10  0.10  0.10  0.10  0.10  0.10  0.10 P₂O₅ 19.15 19.15 19.15 19.15 19.15 19.15 19.15 19.15 19.15 19.15 19.15 19.15 19.15 19.15 19.15 PO₄ 25.62 25.62 25.62 25.62 25.62 25.62 25.62 25.62 25.62 25.62 25.62 25.62 25.62 25.62 25.62 Zn:PO₄  0.05 0.25 0.50 0.13 0.13 0.13 0.13 0.13  0.13  0.13  0.13  0.13  0.13  0.13  0.13 Polyacrylic acid  1.59 1.59 1.59 0.30 1.56 3.00 0.30 1.56  3.00  5.50  7.50 10.00  1.59  1.59   Epoxysilane 1 0.20 0.20 0.20 3.00 3.00  3.00  3.50  6.00   Layer weight P₂O₅ 20-60 mg/m² Peel strength N/4 cm 300- 300- — 250- 400- 400- 250- 400- 300- 250- 200- 150- 400- 250- 150- 650 650 400 800 800 400 800 600 400 300 250 800 400 250

Table 1 illustrates that on all the very different metallic substrate materials, with the inventive aqueous conversion composition, sandwich structures with excellent or good values of peel strength were achieved and that even for nickel-free conversion compositions, very good values were achieved.

The examples B1 to B6 show excellent results, examples B7 to B17 show very good results. The examples B18 to B43 illustrate good results with a peel strength of more than 300 N/4 cm, so that overall good sandwich structures are producible in a broad chemical field of the aqueous conversion compositions. The comparative examples VB44 to VB46 and VB48 show less good results since clearly, for these aqueous compositions, excessively high levels of polyacrylic acid or of silane were added.

Lower layer weights herein typically produced a better peel strength than higher layer weights. The inventive sandwich structures showed an outstanding peel strength. They also met the very high standards of the automotive industry with regard to joining behavior, corrosion-resistance, formability and coatability, as determined in further tests (not shown here).

First tests with the inventive method, revealed that, under the same conditions and without an additional second conversion coating, the galvanized steel sheets Z and ZE can now both be successfully coated and further processed to sandwich structures. The process stability of the preparation of the metallic surfaces, of the aqueous conversion composition and of the conversion coating process was also greater than in the earlier tests, as was revealed by high peel strength values overall. There was found to be a greater bath stability of the conversion composition, which had the effect of more even coatings, which therefore also showed better property values. 

1.-16. (canceled)
 17. A method for producing a sandwich structure on the basis of at least one layer of metallic material and on the basis of at least one layer of organic polymer, characterized in that at least one surface on at least one metallic layer which is to be combined with at least one layer of organic polymer is brought into contact with an aqueous conversion composition which contains: 0.5 to 20 g/l zinc, 0.01 to 10 g/l manganese, 0.01 to 10 g/l aluminum, 0.01 to 1 g/l chromium(III), 0.01 to 5 g/l iron(II), 0.01 to 5 g/l iron(III) and/or 0.01 to 5 g/l magnesium, 0 or 0.01 to 5 g/l of the total as nickel and/or cobalt, 0 or 0.01 to 5 g/l of the total as molybdenum, tantalum, vanadium and/or tungsten, 2 to 100 g/l P₂O₅, which corresponds to 2.68 to 133.8 g/l PO₄, 0.1 to 10 g/l polyacrylic acid, but not more than 25% of the P₂O₅ content of the composition in g/l, and 0 or 0.01 to 3 g/l silane, but not more than 25% of the P₂O₅ content of the composition in g/l, in that a liquid film produced therewith is dried on, in that the metallic layer coated in this manner is cut, if required, and in that the metallic layer coated in this manner is brought into contact with at least one layer on the basis of organic polymer and is combined into a sandwich structure by means of compaction under pressure and/or temperature.
 18. The method according to claim 17, characterized in that the aqueous conversion composition has the following composition: 1 to 10 g/l zinc, 0.5 to 6 g/l manganese, 0.01 to 0.5 g/l aluminum, 0.01 to 0.8 g/l chromium(III), 0.01 to 1 g/l iron(II), 0.01 to 1 g/l iron(III) and/or 0.01 to 1.5 g/l magnesium, 0 or 0.01 to 2.5 g/l nickel, 0 or 0.01 to 2.5 g/l cobalt, wherein the total of nickel and cobalt is 0 or lies in the range from 0.01 to 4 g/l, 0 or 0.01 to 5 g/l of the total as molybdenum, tantalum and/or vanadium, 8 to 60 g/l P₂O₅, which corresponds to 10.72 to 80.28 g/l PO₄, 0.5 to 5 g/l polyacrylic acid, but not more than 25% of the P₂O₅ content of the composition in g/l, and 0 or 0.01 to 3 g/l silane, but not more than 25% of the P₂O₅ content of the composition in g/l and also no content of a complex fluoride of titanium or zirconium.
 19. The method according to claim 17, characterized in that the aqueous conversion composition has the following composition: 2 to 8 g/l zinc, 1 to 5 g/l manganese, 0.01 to 0.2 g/l aluminum, 0.01 to 0.5 g/l chromium(III), 0.01 to 0.5 g/l iron(II), 0.01 to 0.5 g/l iron(III) and/or 0.01 to 0.8 g/l magnesium, 0 or 0.01 to 2 g/l nickel, 0 or 0.01 to 2 g/l cobalt, wherein the total of nickel and cobalt is not more than 2.5 g/l, 0 or 0.01 to 5 g/l of the total as molybdenum, and/or vanadium, 9.5 to 50 g/l P₂O₅, which corresponds to 12.73 to 66.9 g/l PO₄, 0.5 to 3 g/l polyacrylic acid, but not more than 25% of the P₂O₅ content of the composition in g/l, and 0 or 0.01 to 2 g/l silane, but not more than 25% of the P₂O₅ content of the composition in g/l and also no content of a complex fluoride of titanium or zirconium.
 20. The method according to claim 17, characterized in that the method takes place without an activation step with a colloidal titanium phosphate or with a surface conditioner on the basis of phosphate particles.
 21. The method according to claim 17, characterized in that a wet film of the aqueous conversion composition is homogeneously formed on the metallic surface and that the contact time with the aqueous conversion composition until complete drying on is less than 1 minute.
 22. The method according to claim 17, characterized in that the liquid film thereby produced is dried on without being rinsed herein or hereafter with aqueous liquid.
 23. The method according to claim 17, characterized in that a conversion coating with a layer weight of up to 0.4 g/m² is formed.
 24. The method according to claim 23, characterized in that the conversion coating is formed in the form of a microphosphating.
 25. The method according to claim 23, characterized in that the conversion coating is formed largely or entirely amorphous.
 26. The method according to claim 17, characterized in that the at least one layer is made of organic thermoplastic polymer which is optionally fiber-reinforced.
 27. The method according to claim 17, characterized in that the at least one layer of organic polymer is a polymer on the basis of polyamide, polyethylene and/or polypropylene, which is optionally made of thermoplastic plastics and/or is also fiber-reinforced.
 28. A sandwich structure produced with a method according to claim
 17. 29. The sandwich structure according to claim 28, characterized in that the content of polyacrylic acid and/or its reaction products in the dried-on and/or the dried-on and, during compaction, thermally loaded, conversion coating is 0.05 to 15% by weight of the conversion coating.
 30. The sandwich structure according to claim 28, characterized in that the content of at least one silane and/or its/their reaction products in the dried-on and/or the dried-on and, during compaction, thermally loaded conversion coating is 0.01 to 15% by weight of the conversion coating.
 31. The sandwich structure according to claim 28, characterized in that it is at least once respectively coated, deformed, glued, compressed and/or otherwise joined.
 32. Use of the sandwich structure according to claim
 28. 33. Use of the sandwich structure produced in accordance with the method according to the claim 17 in motor vehicle construction, in aircraft construction, in space travel, in apparatus construction, in machine construction, in building construction, in furniture production or as structural elements. 